She did ask Harry if Draco was in the castle; Harry responded yes, and Narcissa then told Voldemort that Harry was dead. She knew the only way she would be able to return to the castle to search for Draco would be as part of Voldemort's conquering army, and so she lied, in order to compel Voldemort back to the castle, because she knew he wouldn't be able to resist the opportunity to lord his victory over the Order and its supporters. And she was right :)
‘Is Draco alive? Is he in the castle?’
The whisper was barely audible; her lips were an inch from his ear, her head bent so low that her long hair shielded his face from the onlookers.
‘Yes,’ he breathed back.
Still feigning death on the ground, he understood. Narcissa knew that the only way she would be permitted to enter Hogwarts, and find her son, was as part of the conquering army.
She no longer cared whether Voldemort won.
Deathly Hallows - chapter 36 - The Flaw In the Plan
For what it's worth, the Malfoy escaped imprisonment in Azkaban for their Death Eater activities because they defected from the Death Eaters at the end and fought on Harry's side, albeit undoubtedly for their own personal gain.